The present invention relates to a mobile communication system using the CDMA (Code Division Multiple Access) scheme and a transmission power control method therefor and, more particularly, to a transmission power control system and method in malfunction of the transmitting unit of a base station and a decrease in transmission power.
Along with the recent progress in electronic communication technology, mobile communication systems such as automobile telephones and portable telephones are rapidly proliferating. At the same time, the mobile communication scheme is also shifting from the TDMA (Time Division Multiple Access) to the CDMA.
The cellular mobile communication system using the CDMA scheme generally has the following advantages.    1) The system has high resiliency against interference such as radio interference or disturbance.    2) The system rarely interferes with another system because the power spectrum density is low.    3) The system is excellent in security because the power spectrum density is low.    4) The system can realize satisfactory privacy protection by spreading codes.    5) Use of different spreading codes allows multiple access.    6) Overload communication is possible.
However, the system has not only the advantages but also problems. One of the problems is the interference problem. A typical interference problem is the near-far problem. The near-far problem is a phenomenon in which during communication between a base station and a desired mobile station at a remote site, another interfering mobile station near the base station gives large disturbance to the desired mobile station that is communicating. Such an interference problem is not limited to the CDMA. Even for a conventional mobile communication system, this problem is known as interchannel interference (single channel interference/neighboring channel interference). This problem is especially conspicuous in the CDMA because a number of mobile stations share a single frequency.
To solve the interference problem, a mobile communication system using the CDMA scheme has various transmission power control means for reducing the interference amount. Transmission power control includes reverse-link (up-link) transmission power control from a mobile station to a base station and forward-link (down-link) transmission power control from a base station to a mobile station.
Reverse-link transmission power control based on the TIA/EIA (Telecommunication Industry Association/Electronic Industry Association) IS-95 standard will be described next. Reception quality measurement in reverse-link transmission power control is done by the receiving section of a base station. That is, a base station measures the reception quality of a radio signal sent from a mobile station, and if the reception quality is more than a predetermined threshold value, instructs the mobile station by a power control bit to reduce the transmission power. If the reception quality is less than the predetermined threshold value, the base station instructs the mobile station by a power control bit to increase the transmission power. This reverse-link transmission power control is called a closed-loop transmission power control scheme.
However, the conventional reverse-link transmission power control scheme has the following problems. As shown in FIG. 4, in the conventional scheme, the mobile station in the standby state has completed synchronization acquisition with the base station and is set in the idle state while performing control channel transmission/reception such as field measurement in the peripheral zone and position registration according to a change in position (step S11). The mobile station in this state has already received from the base station system parameters related to the mobile station and stored them when powered on.
System parameters related to a mobile station are a plurality of constants that define the system conditions of the mobile station in the mobile communication system, and are determined in advance in designing the entire system including the base station. The system parameters related to the mobile station are stored in the base station together with system parameters related to the base station. Every time a new mobile station is activated, the system parameters related to the mobile station are transferred from the base station to the mobile station via a control channel.
Assume that the transmitting unit (TX) of the base station malfunctions to make the forward-link transmission power of the base station lower than a predetermined steady transmission output (step S12), and then, the mobile station in the standby state starts originating or terminating operation.
For the reverse-link control channel at the mobile station that is starting originating or terminating operation, since transmission power control operation by a closed loop is not activated, transmission power control operation by the mobile station is activated. In the basic operation of transmission power control performed by the mobile station, near the mobile station (at a point where the reception field strength of the forward-link control channel at the mobile station is high), the reverse-link transmission power of the mobile station is set to be low in order to reduce the reverse-link interference amount at the base station. On the other hand, at a point far from the base station (at a point where the reception field strength of the forward-link control channel at the mobile station is low), the reverse-link transmission power of the mobile station is set to be high in order to ensure the speech communication quality.
Hence, the reverse-link control channel at the mobile station that is starting originating or terminating operation is set to transmission power whose value is the sum of a variable value inversely proportional to the reception field strength of the forward-link control channel from the base station and a fixed value based on the transmission power initial constant of the system parameters related to the mobile station (step S13). The set reverse-link transmission power is radiated to the control channel between the mobile station and the base station (step S14).
That is, the reception field strength value of the forward-link control channel decreases in linkage with the reduced transmission power of the base station, and the transmission power value of the reverse-link control channel at the mobile station increases in inverse proportion to that reception field strength. For this reason, the amount of interference to the base station and neighboring base stations increases.
If the communication shifts to the traffic channel (TCH) to perform actual speech communication, transmission power control processing by a closed loop starts, and the reverse-link transmission power is controlled to an appropriate value (steps S15 and S16).
As described above, when the transmission power of the base station abruptly decreases, the reception field strength of the forward-link control channel abruptly decreases, and consequently, the mobile station which is standing by in the service area of the mobile station erroneously recognizes that the mobile station has moved away from the base station in an instant. For this reason, the mobile station radiates transmission power more than the appropriate value in the reverse-link control channel in starting originating or terminating operation.